The infinite amount of space within something in which items are stored. Inventories in video games are a prime example, as the character is somehow able to hold a metric fuckton of stuff without any sign of even a bag on him.

Guy 1: "Dude, 500 pokéballs?! How do you even fit all of that in one pocket?!"

1. The place where hammers live. Often an unseen, extradimensioal space just big enough to store one large wooden mallet, used to hit perverts over the head. From anime.
2. Any such hidden location where an object, usually violent in nature, comes from unexpectedly

1. "Did you see Akane? Kuno was getting gropy and then out of hammerspace he got whacked on the head by her mallet."
2. "These action movies are too unrealistic--everyone has hammerspace."

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Hammer space is the place where anime girls get their weapons from (normally, as the name suggests, hammers). This term was coined probably because anime-watchers were always wondering where on earth the girls carry their huge mallets or hammers. They seem to appear out of nowhere...well, that nowhere is Hammer Space!

With an enraged look in her eyes, she reached back behind herself and pulled a giant mallet out of Hammer Space, then started to beat the perpetrator unrelentingly.